Glass fabric reinforced mica tape and process for production thereof



March 25, 1969 R. s. ASHPOLE ET AL 3,434,872

, GLASS FABRIC REINFORCED MICA TAPE AND PROCESS FOR PRODUCTION THEREOF Filed Aug. 2. 1965 Sheet of 2 March 25, 1969 ASHPOLE ET AL 3,434,872

GLASS FABRIC REINFORCED MICA TAPE AND PROCESS FOR PRODUCTION THEREOF Filed Aug. 2. 1965 Sheet 2 of 2 United States Patent U.S. Cl. 117'126 6 Claims ABSTRACT OF THE DISCLOSURE In a process for making reinforced mica tape, mica is deposited electrophoretically from a suspension of mica pulp in liquid onto a glass fibre fabric reinforcing tape.

This invention relates to insulating materials and to processes for production thereof.

According to this invention in one aspect, a process for the production of an insulating material in tape or sheet form includes the steps of forming a suspension of flakes and/ or fibres of the insulating material in a liquid, passing a porous backing material through said suspension between an anode and a cathode, in close proximity to or in contact with the anode and spaced away from said cathode, and applying a D.C. electrical potential between said anode and said cathode so as to cause flakes or fibres to be deposited on said backing material.

According to this invention in another aspect, an insulating material in tape or sheet form include an openweave reinforcing material and the interstices of the weave contain flakes or fibres of an insulating material. Preferably the interstices contain mica flakes.

A number of examples of processes in accordance with the invention will now be described by way of example with reference to the accompanying drawings, of which:

BIG. 1 illustrates one example of a process in accordance with the invention.

FIG. 2. shows a modification of FIG. 1.

FIG. 3 illustrates a modified process in accordance with the invention.

FIG. 4 shows the product resulting from the process of FIG. 3.

FIG. 5 shows a further modified process in accordance with the invention, and

FIG. 6 shows the product resulting from the process of FIG. 5.

Referring to FIG. 1, there is shown a bath 11 containing a suspension of mica flakes in a mixture of water and acetone, An anode 12 and a cathode 13 are partially immersed in the bath and are spaced apart. Means (not shown) is provided for applying a DC. potential between the anode and the cathode. Glass fabric tape 14 passes from a takeoff reel 15 over suitable rollers 16 through the suspension and over the anode 12. The tape 14 makes contact with the anode 12 over the greater part of the depth of the bath and for some distance above the surface of the suspension. The tape 14 passes over a pulley 17, which is preferably partially evacuated internally, for reasons which will become apparent, and the tape then passes to a drier 18 and take-up spool (not shown). The cathode 13 is fixed at an appropriate distance from the anode, and the glass fabric tape 14 is between the two electrodes. The application of the DC. voltage between the electrodes causes the mica flakes in the suspension to drift towands the anode and therefore to collect on the surface of the tape. The quantity of the flakes collected depends on the time for which the tape remains in the bath.

The process has been found to operate successfully over a wide range of electric field intensities; the preferred field intensity between the anode and the cathode is between 0.05 volt per mil and 0.8 volt per mil. The resistivity of the liquid in the bath is preferably greater than 20K ohm cm.

The suspension is circulated through the bath 11 from a reservoir 19 containing the bulk of the suspension, the suspension being delivered from the reservoir 19 to the bath 11 through a pipe 20 by means of a circulating pump 21 and being returned from the bath 11 through a pipe 22 to the reservoir. This circulation prevents the settling of the flakes in the suspension and also facilitates the addition of mica flake at intervals to the liquid. in the reservoir, enabling the mica content of the suspension to be maintained. An alternative method of preventing settling is to arrange for the suspension in the bath to be stirred.

In a specific example, the composition of the suspension was as follows:

Gm. Mica flakes 10 Ben-a-gel in distilled water /2 percent solution) 100 Acetone Ben-a-gel is a propietary suspending agent consisting of highly purified magnesium montmorillonite and is obtainable -from F. W. Berk & Co. Ltd, Berk House, 8 Baker Street, London W. 1.

iIn the example glass fabric tape 1 inch wide and 0.005 inch thick was passed through the bath at a speed of up to 8 feet per minute. The depth of the suspension in the bath was approximately 6 inches and the separation between the electrodes was 1 inch. A current of 0.5 amp was passed between the electrodes at volts DC. It was found that mica was deposited on the glass fabric tape to a thickness of 0.002. inch.

The use of a partially-evacuated pulley having a porous or mesh surface has been found to help in drying the coated tape. Alternatively, the tape may be passed over a stationary vacuum box. The coated tape can be allowed to dry in air or may be dried by any convenient means.

The dried tapes can be sprayed with a suitably diluted resin solution to assist in bonding the mica flakes and the tape, or a resin may be included in the suspension, provided that it does not interfere with the electro-deposition of the flakes.

Referring now to FIG. 2, in place of the stationary anode 12 shown in FIG. 1 there may be provided an anode 32 in the form of an endless belt passing over pulleys 33.

Referring to FIG. 3, there is shown a process for producing a mica tape reinforced by open weave glass tape, instead of the use of the glass fabric backing tape. The use of an open weave tape reduces the ratio of glass to mica in the final product, it is also found that the mica flakes adhere better to the open weave glass tape.

The processes of the invention were developed for the preparation of mica tape from mica flake reconstituted from scrap mica. Various methods of reconstituting mica are kn own. It has been found that a particular property of such mica flakes is their ability to bond together due to natural cohesive forces, without the addition of bonding agents. It is this self-bonding effect of the reconstituted mica flakes which enables the reinforcing open weave glass tape to remain in place within the body of the mica tape.

The open weave tape 34 is fed through the coating bath 11 at the same time as the glass fabric tape 14, from a take-off roller 3-5, and passed through the suspension a-djacent to the glass fabric tape 14 and between the glass fabric tape and the cathode 1.3. During the deposition of the mica flakes as described above, the open Weave tape 34 becomes embedded, since some of the mica flakes penetrate the open weave and fill in the interstices of the Wave.

After drying the coated tape, the glass fabric backing tape 14 is removed, leaving the mica tape reinforced by the open weave glass fabric. In this modification of the process, the glass fabric backing tape can be in the form of an endless belt, as it does not form part of the final product.

The product is illustrated in FIG. 4, showing the removal of the backing tape.

Referring to FIG. 5, the open weave fabric can be embedded to a greater depth in the mica flakes by the further modification shown. The open weave tape 44 is taken from the take-off reel 45 and passes round a pulley 46 in the coating bath 11 so arranged that the open weave tape 44 is separated from the backing tape 14 at the lower end of the anode 12, and the build-up of mica flakes on the backing tape 14 reaches the open weave fabric at an intermediate point along the anode and further build-up of mica flakes then takes place in the interstices of the open weave fabric and on the side of the fabric toward the cathode.

The resulting produce is illustrated in FIG. 6, which also shows the removal of the glass fabric backing tape 14.

A bonding resin may be used if desired in each of the modifications described. Ordinary mica splittings produced by the breaking up of block mica do not have such a marked self-bonding action as reconstituted mica flakes, and thus where this type of material is used a bonding agent will probably be found necessary.

It Will also be appreciated that the processes described may be used in respect of flake insulating materials other than mica, and that the backing and reinforcing fabric need not necessarily be of glass. In the majority of cases the flake materials will not have a marked self-bonding action, and thus a bonding agent will be required.

What we claim as our invention and desire to secure by Letters Patent is:

1. A process for making mica tape reinforced with glass fibre, wherein a glass fibre fabric reinforcing strip is passed through a suspension of mica pulp, in liquid, there being an anode immersed in said suspension and arranged adjacent and parallel to the reinforcing strip, and a DO electric current being applied between the anode and a cathode which is immersed in the suspension with the reinforcing strip between cathode and anode, so that mica is deposited electrophoretically on the reinforcing strip.

2. A process according to claim 1, wherein the anode comprises a continuously-moving electrically-conducting endless member.

3. A process according to claim 1, wherein said reinforcing strip is of open-weave glass fibre fabric so that mica can lodge between the threads thereof, a backing strip impervious to the mica being passed between the anode and the reinforcing strip.

4. A process according to claim 3, wherein said backing strip is of close-Weave .glass fibre fabric.

5. A process according to claim 3, whreein the reinforcing strip is separated from the backing strip by a distance sufficient to allow mica to be deposited on the side of the reinforcing strip nearest the backing strip.

6. A mica tape reinforced with open-weave glass fib re fabric, wherein all the external surfaces of the tape are formed of mica layers keyed together by further mica between the threads of said fabric.

References Cited UNITED STATES PATENTS 1,641,322 9/1927 Czapek et a1 204181 X 2,393,068 1/1946 Ruben 204-181 3,001,571 9/1961 Hatch 162-l52 X FOREIGN PATENTS 1,081,091 5/1960 Germany.

1,137,372 9/1962 Germany.

- ALFRED L. LEAVITT, Primary Examiner.

I. H. NEWSOME, Assistant Examiner.

US. Cl. X.R. ll7--76; 204-181 

